docs: def build_index():
      """Load information from the data directory
  
      Returns
      -------
      A namedtuple with three fields: nationalities cities countries
      """
  
      nationalities = read_table(get_data_path('nationalities.txt'), sep=':')
  
      # parse http://download.geonames.org/export/dump/countryInfo.txt
      countries = read_table(
          get_data_path('countryInfo.txt'), usecols=[4, 0], skip=1)
  
      # parse http://download.geonames.org/export/dump/cities15000.zip
      cities = read_table(get_data_path('cities15000.txt'), usecols=[1, 8])
  
      # load and apply city patches
      city_patches = read_table(get_data_path('citypatches.txt'))
      cities.update(city_patches)
  
      Index = namedtuple('Index', 'nationalities cities countries')
      return Index(nationalities, cities, countries)